Fast fFood

Titles usually use title case for capitalization

Nowadays, fast- foods has been diffusspreading worldwide: we can find a McDonald's every kilometer!

"fast-food" is already implied to be plural, so it shouldn´t have an "s" here. You could also say "fast-food restaurants have been spreading..."

"diffuse" isn't technically wrong, but it does not sound natural to me in this context.

This has some benefits; firstly, we don't have to worry if we're juggling tasks and don't have time to cook, as we can grab food from a fast-food restaurant and eat it quickly.

"juggling" on its own implies the sport where a person throws balls up in the air, so the extra term helps clarify.

If you say "a fast-food," that is treating it like a noun, but that can only really be done if you just say "fast-food" without the "a". Two examples:

1. we can grab food from a fast-food restaurant and eat it quickly.

OR

2. we can grab fast-food and eat it quickly.

either works, but the second one is even more concise

Secondly, as they're very widespreaded, you can always stop and eat something every time you're hungry.

"they're" has to refer to a plural noun, so that means the preceding sentence would have to say "fast-food restaurants/places"

alternatively, you could say "as it is very widespread" if you are talking not about "fast-food restaurants," but rather "fast-food"

ThenNext, they are very cheap, so you can hang out with friends and spend some quality time without spending too much!

"then" works best as a temporal sequence transition word (I went to the store. Second, I got my groceries. Then, I paid for it all.)

However, they have a lot of cons: in the first place, as everyone knows, they are not healthy at all, and can cause problems to your well-being.

they are not healthy at all and can cause problems to your well-being.

OR

they are not healthy at all, and they can cause problems to your well-being.

SecondlyAlso, they can cause addictions: this is becoming a very popular problem in the USA, with too muchany people considered obese and overweight.

"also" avoids using "secondly" twice

And, finally, we are losing the real perconception of food: nowadays, food is constantly filled with additives and conpreservantives, making us wastemiss the real taste of food.

"perception" means "how people see it," but "concept" means more like "how it should be," which I think is closer to your meaning.

"conservants" seems to mean "preservatives"

"waste" usually means to have something and not use it well, but since it seems like you are saying that we do not really have the real taste anymore, "miss" or even "lose" could be better

In conclusion, fast-food is are well-structured necessity, but abusing them will only cause damages.

same thing here:

fast-food restaurants are ...

or

fast-food is...

Fast fFood

This is a minor issue, but all words in a title are generally capitalised.

Nowadays, fast- foods has been diffusing worldwide: we can find a McDonald's (for) every kilometer!

(1) When referring to fast food in general, "fast food" is an uncountable noun. "Fast foods" (countable) would refer instead to types of fast foods: for instance, you can say that burgers and fries are examples of fast foods. Here, "fast food" (uncountable) would be more appropriate.
(2) You can also say "fast food" (without the hyphen). What's important is that you be consistent with your hyphenation. You should either hyphenate all instances of "fast food", or don't at all.
(3) Strictly speaking, "McDonald's" is the correct name, not "McDonald".

This has some benefits; firstly, we don't have to worry if we're juggling work and don't have time to cook, as we can grab food from a fast-food restaurant and eat it quickly.

You need to clarify what it is that's being juggled.

Secondly, as they'reit's very widespreaded, you can always stop by a fast-food restaurant and eat something everytime you're hungry.

(1) Alternatively: "since it's very widespread".
(2) As aforementioned, in this context, we generally use "fast food" as an uncountable noun. For this reason I have changed "they're" to "it's".
(3) "Widespreaded" is not a word; "widespread" is the correct spelling.
(4) The phrase is "stop by (a place)".

Then, they areirdly, it is very cheap, so you can hang out with friends and spend some quality time without spending too much!

Given the use of "firstly" and "secondly", it is natural to use "thirdly" as well.

However, theyfast-food haves a lot of cons: in the first place, as everyone knows, they areit is not healthy at all, and can cause problems to your well-being.

(1) It has been a while since you last mentioned the subject, so I reiterated the phrase "fast-food". It is good writing to do this every now and then.
(2) "Everyone" is singular, so "knows" is expected.

Secondly, ithey can cause addictions: this is becoming a very popularrevalent problem in the USA, with too muchany people considered obese and overweight.

(1) If you do not wish to use "secondly" again, considering that you'd already used it before, you can also consider: "additionally", "moreover", "furthermore", "in addition", etc.
(2) "Popular" is not used this way. Words you can use are: "prevalent", "widespread", "pervasive", etc.
(3) "Much" is used only with uncountable nouns. For countable nouns such as "people", we use "many".

And, finally, we are losing the real perception of food: nowadays food is constantly filled with additives and conpreservantives, making us wastlose the real taste of food.

In conclusion, fast-foods are well-structured necessityies, but abusing them will only cause damages.

(1) I'm not certain what you meant by "well-structured" here, so I just removed it. Perhaps you can provide some clarification, in the form of native text or otherwise.
(2) Alternatively: "In conclusion, fast-food is a necessity" (uncountable).
(3) Generally, "damage" is also an uncountable noun.